/*
 * Copyright (c) Kumo Inc. and affiliates.
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#pragma once

#include <cassert>
#include <new>
#include <type_traits>
#include <utility>

#include <melon/traits.h>
#include <melon/utility.h>

namespace melon {
    /***
     *  Indestructible
     *
     *  When you need a Meyers singleton that will not get destructed, even at
     *  shutdown, and you also want the object stored inline.
     *
     *  Use like:
     *
     *      void doSomethingWithExpensiveData();
     *
     *      void doSomethingWithExpensiveData() {
     *        static const Indestructible<map<string, int>> data{
     *          map<string, int>{{"key1", 17}, {"key2", 19}, {"key3", 23}},
     *        };
     *        callSomethingTakingAMapByRef(*data);
     *      }
     *
     *  This should be used only for Meyers singletons, and, even then, only when
     *  the instance does not need to be destructed ever.
     *
     *  This should not be used more generally, e.g., as member fields, etc.
     *
     *  This is designed as an alternative, but with one fewer allocation at
     *  construction time and one fewer pointer dereference at access time, to the
     *  Meyers singleton pattern of:
     *
     *    void doSomethingWithExpensiveData() {
     *      static const auto data =  // never `delete`d
     *          new map<string, int>{{"key1", 17}, {"key2", 19}, {"key3", 23}};
     *      callSomethingTakingAMapByRef(*data);
     *    }
     */

    struct factory_constructor_t {
        explicit factory_constructor_t() = default;
    };

    constexpr factory_constructor_t factory_constructor{};

    template<typename T>
    class Indestructible final {
    public:
        template<typename S = T, typename = decltype(S())>
        constexpr Indestructible() noexcept(noexcept(T()))
            : storage_{std::in_place} {
        }

        /**
         * Constructor accepting a single argument by forwarding reference, this
         * allows using list initialization without the overhead of things like
         * std::in_place, etc and also works with std::initializer_list constructors
         * which can't be deduced, the default parameter helps there.
         *
         *    auto i = melon::Indestructible<std::map<int, int>>{{{1, 2}}};
         *
         * This provides convenience
         *
         * There are two versions of this constructor - one for when the element is
         * implicitly constructible from the given argument and one for when the
         * type is explicitly but not implicitly constructible from the given
         * argument.
         */
        template<
            typename U = T,
            std::enable_if_t<std::is_constructible<T, U &&>::value>* = nullptr,
            std::enable_if_t<
                !std::is_same<Indestructible<T>, remove_cvref_t<U> >::value>* =
                    nullptr,
            std::enable_if_t<!std::is_convertible<U &&, T>::value>* = nullptr>
        explicit constexpr Indestructible(U &&u) noexcept(
            noexcept(T(std::declval<U>())))
            : storage_{std::in_place, std::forward<U>(u)} {
        }

        template<
            typename U = T,
            std::enable_if_t<std::is_constructible<T, U &&>::value>* = nullptr,
            std::enable_if_t<
                !std::is_same<Indestructible<T>, remove_cvref_t<U> >::value>* =
                    nullptr,
            std::enable_if_t<std::is_convertible<U &&, T>::value>* = nullptr>
        /* implicit */ constexpr Indestructible(U &&u) noexcept(
            noexcept(T(std::declval<U>())))
            : storage_{std::in_place, std::forward<U>(u)} {
        }

        template<typename... Args, typename = decltype(T(std::declval<Args>()...))>
        explicit constexpr Indestructible(Args &&... args) noexcept(
            noexcept(T(std::declval<Args>()...)))
            : storage_{std::in_place, std::forward<Args>(args)...} {
        }

        template<
            typename U,
            typename... Args,
            typename = decltype(T(
                std::declval<std::initializer_list<U> &>(), std::declval<Args>()...))>
        explicit constexpr Indestructible(std::initializer_list<U> il, Args... args) noexcept(
            noexcept(T(
                std::declval<std::initializer_list<U> &>(), std::declval<Args>()...)))
            : storage_{std::in_place, il, std::forward<Args>(args)...} {
        }

        template<typename Factory>
        constexpr Indestructible(factory_constructor_t, Factory &&factory) noexcept(
            noexcept(factory()))
            : storage_(factory_constructor, std::forward<Factory>(factory)) {
        }

        Indestructible(Indestructible const &) = delete;

        Indestructible &operator=(Indestructible const &) = delete;

        T *get() noexcept { return reinterpret_cast<T *>(&storage_.bytes); }

        T const *get() const noexcept {
            return reinterpret_cast<T const *>(&storage_.bytes);
        }

        T &operator*() noexcept { return *get(); }
        T const &operator*() const noexcept { return *get(); }
        T *operator->() noexcept { return get(); }
        T const *operator->() const noexcept { return get(); }

        /* implicit */
        operator T &() noexcept { return *get(); }
        /* implicit */
        operator T const &() const noexcept { return *get(); }

    private:
        struct Storage {
            aligned_storage_for_t<T> bytes;

            template<typename... Args, typename = decltype(T(std::declval<Args>()...))>
            explicit constexpr Storage(std::in_place_t, Args &&... args) noexcept(
                noexcept(T(std::declval<Args>()...))) {
                ::new(&bytes) T(std::forward<Args>(args)...);
            }

            template<typename Factory>
            constexpr Storage(factory_constructor_t, Factory factory) noexcept(
                noexcept(factory())) {
                ::new(&bytes) T(factory());
            }
        };

        Storage storage_{};
    };
} // namespace melon
